The invention relates to a device with microbubble-induced superhydrophobic surfaces for drag reduction and biofouling prevention and a device for biofouling prevention.
Overcoming resistances is the basic requirement for ships. FIGS. 1A and 1B show a ship moving at a speed V. The resistances to the ship include about 85%-95% of frictional resistance RF and wave making resistance RW, about 3%-5% of vortex flow loss RE, and 2% (for a low-speed ship) to 10% (for a high-speed ship) of air resistance RA. The affect of the frictional resistance RF to a low-speed ship (about 15 nautical miles per hour) is significant and may be up to 90%.
Thus, how to reduce the frictional resistance RF so as to reduce the energy consumption for moving a ship has been an important issue in fluid dynamic research of ships. Many methods for reducing the frictional resistance RF, such as surface structure with riblets for biomimetic drag reduction, artificial microblowing, microbubble injection, surface coating for reducing drag, or nanotechniques, have been proposed. Among these methods, the surface coating is the most convenient and most economical but least eco-friendly whereas the microbubble injection is the most eco-friendly and provides excellent drag reducing effect.
The earliest microbubble drag reducing technique was disclosed by M. Mc Cormick and R. Bhattacharya in 1973. In 1985, N. Madavan obtained drag reducing effect up to 80% in microbubble drag reduction experiments in 1985 by passing compressed air through porous plates, which was confirmed by a research of H. Kato in 1994 by using an identical experimental apparatus. Thus, microbubble drag reduction devices passing compressed air through porous plates have been widely used in research institutes throughout the world.
However, the drag reducing effect of the microbubble drag reduction devices on an actual ship is less than 2%. Reasons of such a tremendous difference could be:
1. The diameters of the bubbles generated by the porous plates are so large that the bubbles overlap and become larger due to buoyant effect. The larger bubbles could adhere to the hull surface and increase the resistance to the hull. On the other hand, the larger bubbles could escape and, thus, could not remain in the effective boundary layer. As a result, the larger bubbles move to the water surface and, thus, fail to provide the drag reducing effect.2. The area of the hull surface of the actual ship covered by the porous plates is too small (see FIG. 2A), such that the microbubbles could not cover most of the hull surface below the water surface.3. When the amount of air supplied to the porous plates is increased, the microbubbles are apt to overflow out of the boundary layer under the action of vertical velocity that is created while injecting the microbubbles into the flow field around the hull, leading to adverse affect to the drag reducing efficiency.
FIGS. 2A and 2B show an example of a conventional microbubble drag reduction device mounted on a ship. The microbubble drag reduction device includes a blower A mounted on a deck for compressing air. The compressed air is delivered through an air pipeline (not shown) to a porous plate C mounted on a bottom plate B of the ship. Microbubbles are formed and injected into the flow field of the seawater. The porous plate C is generally located adjacent to the how to obtain the maximal drag reducing effect. However, two side boards D of the bottom plate B intended for restraining the microbubbles and preventing overflow of the microbubbles cause low utility ratio of the microbubbles. In addition to the low drag reducing efficiency, the conventional microbubble drag reduction device incurs many structural safety problems including the risks of sealing of the hull, corrosion of the hull, and biofouling of marine organisms. Further, the bubbles injected into the seawater reduce the propelling efficiency of the vanes of the propeller by about 3% and cause surface cavitation corrosion of the vanes.
Thus, in actual practice of drag reduction of a ship, it is necessary to increase the hull surface area covered by bubbles, to avoid the bubbles from becoming larger due to overlapping, and to prevent the bubbles from overflowing out of the boundary layer on the premise of preventing adverse affect to the structural safety of the ship.
In view of the many disadvantages of the microbubble drag reduction device passing compressed air through porous plates, R. Wedin of France generates microbubbles by electrolysis. The microbubbles have a diameter much smaller than (about ⅛ of) the diameter of the microbubbles generated by the porous plates made of porous material. Furthermore, after formation of nucleuses of the microbubbles on the electrode surface, the flow field of the hull can directly carry the microbubbles to a downstream side of the hull, such that the microbubbles can be distributed over the hull surface below the water surface while avoiding the problem of deviation away from the boundary layer under the action of vertical velocity. The drag reducing effect can be maximized.
However, the amount of air generated is too small, and the energy conversion ratio of the energy saved in reducing drag to the inputted electricity is small. If it is intended to generate a large amount of microbubbles, the energy consumed will be significantly increased, and heat convection resulting from increased electric current will occur.
On the other hand, some marine organisms love to adhere to the hull surface that is continually submerged below the water surface, increasing the roughness of the hull surface and increasing the overall weight of the hull up by to 75% and, thus, leading to reduction in the speed of the ship. Taking a ship moving at a speed of 15 nautical miles per hour as an example, in a case that the marine biofouling is serious, the resistance to the ship may be increased by up to 80%, the fuel consumption may be increased by up to 86%, and the engine will discharge a large amount of exhaust gas. Furthermore, the marine organisms adhered to the hull surface below the water surface increase the corrosion speed of the steel sheets of the hull and may even risk the safety of the machine and maneuver of the ship. Further, the marine organisms inhabit any artificial structure or tool, such as set nets, anchor chains, and cooling pipelines of generators, speeding up metal corrosion or increasing the flow resistance of water.
In a conventional technique, anti-fouling paint is coated on the hull surface to prevent marine biofouling on the hull surface below the water surface. Among various anti-fouling paint, self-polishing copolymer tributyltin (TBT) paint is commonly used, because it is more toxic but stable and may last up to 60 months. However, self-polishing copolymer TBT paint is detrimental to the ocean environment, the marine creatures, and human bodies. Therefore, the International Maritime Organization (IMO) has forbidden use of self-polishing copolymer TBT paint on hulls since Jan. 1, 2008 to mitigate damage to the ocean environment. Several substitutes have been developed in the art, such as:
1. Copper-based anti-fouling paint: The toxicity of copper-based anti-fouling paint is less than TBT 1000 times. However the cooper-based anti-fouling paint is effective only on animal organisms. Weedicide must be added to achieve the antifouling purposes. Thus, use of the copper-based anti-fouling paint containing weedicide may be controlled in the future in view of the potential new threat to the environment by the concealed toxicity.2. Tin-free anti-fouling paint: The effective period of tin-free anti-fouling paint is about 24 months, which is shorter than TBT. Tin-free anti-fouling paint is suitable for ships that require frequent maintenance in dock, such as passenger ships.3. Non-stick coatings: Non-stick coatings are biocide free and have sleek surfaces to cause difficulty in adherence of contaminants and to allow easy removal of the adhered contaminants. Non-stick coatings are suitable for high-speed ships moving at a speed higher than 30 knots and have small frictional resistance. However, repair of broken non-sticking coatings is not easy.4. Application of external electric current by conductive paint or conductive material: Although this method is more effective than tin-free antifouling paint, the cost is high and damage occurs easily. Furthermore, this method consumes considerable electricity and increases the possibility of corrosion of the hull.
In the above techniques, the antifouling paint (whether non-toxic paint or conductive paint) must be coated on the hull surface through use of toluene containing volatile organic compounds (VOC) that are detrimental to the environment.
Further, in conductive paint developed by Mitsubishi Heavy Industries of Japan, disclosed in Taiwan Patent No. 128362 issued in 2007 and entitled “SYSTEM AND METHOD FOR INHIBITING MARINE BIOFOULING BY CONDUCTIVE RUBBER COATING”, and disclosed in Taiwan Patent No. 514680 issued in 2002 and entitled “SYSTEM AND METHOD FOR ANTIFOULING”, extremely toxic chlorine gas obtained from electrolysis is used in inhibiting marine organisms.
In an exception, T. Nakayama of Japan alternatively applied direct current of 1.0 V and −0.6V (slightly lower than the voltage for obtaining chlorine gas by electrolysis) on a film of titanium nitride (TiN) and found effective inhibition of marine biofouling after 279 days of experiment in seawater. The inhibiting mechanism was based on micro electric current that destroys the micro organisms on which the marine organisms feed. Thus, parasitism of microbial films could be inhibited, avoiding adherence and growth of marine organisms including barnacles, tube worms, and oysters. Thus, this antifouling method is eco-friendly. However, preparation of the titanium nitride film requires expensive small-area reactive magnetron sputtering or expensive small-area radio-frequency arc spray.